1. Field of the Invention
The present invention relates to a wireless communication system. More particularly, the present invention relates to an apparatus for filtering an analog baseband signal in a multi-mode and multi-band wireless transceiver and a method for controlling the apparatus.
2. Description of the Related Art
In a receiver of wireless communication, an analog filter is used to filter unnecessary noise from a signal that is demodulated into a baseband by a mixer and to select a signal of a desired channel. In the analog filter, an accurate cut-off frequency setting has a significant influence upon system performance.
Generally, a filter has an input-to-output gain value which changes as a frequency value increases, and the filter has also a pass band and a stop band. A cut-off frequency fc refers to a boundary frequency that is between the pass band and the stop band. A Low Pass Filter (LPF) defines a cut-off frequency fc, which is a frequency having a gain value which is 3 dB lower than a gain value of a direct current or a low frequency in the pass band. The cut-off frequency fc is determined by a feedback resistor and a feedback capacitor, both of which are used in the analog filter.
A baseband used in mobile communication systems covers a very large range of frequencies from a bandwidth of 100 kHz for 2nd Generation (2G) communication systems to a bandwidth of 20 MHz for 3rd Generation (3G) or 4th Generation (4G) communication systems, wherein the highest bandwidth of the baseband is about 100 times the lowest bandwidth. A multi-mode mobile terminal, which uses a 2G mode for voice communication and uses a 3G or 4G (hereinafter, ‘3G/4G’) mode for data communication, should have a multi-mode and multi-band wireless transceiver that uses an analog baseband filter that is capable of supporting the foregoing diverse bandwidths of the baseband.
However, a resistor value, which may also be referred to as a resistance, and a capacitor value, which may also be referred to as a capacitance, which determine the cut-off frequency fc of the analog baseband filter change according to temperature and process conditions, and the resistance and capacitance are difficult to accurately estimate, such that in an actual environment, the cut-off frequency fc may be different from a target value. Hence, the cut-off frequency fc is corrected by controlling a variable resistor and/or a variable capacitor with a digital algorithm, and an error of correction needs to be less than 4%.
The cut-off frequency fc is inversely proportional to a product of a resistor value and a capacitor value, such that in order to process a signal in a low band, such as 2G, a resistor and a capacitor having very large values are needed, and accordingly, an area and/or physical size of the analog filter may increase. A capacitor for processing a low band of 2G may be several times larger in size than that in a 3G/4G, thus increasing a circuit area of the analog filter several times. As such, when the 3G/4G mode operates, the circuit area of the analog filter significantly increases due to the idle 2G mode, thus increasing the processing cost with respect to power, computation and other similar resources. Moreover, as the circuit area increases, a path length of a signal travelling through the circuit increases, thus increasing an error of the signal and noise, and thus degrading characteristics of the signal.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.